The invention relates to a proton conductive membrane for electrochemical applications, a polymer for the manufacture of the membrane and the use of the membrane in a fuel cell.
Fuel cells are considered to be promising low-emission alternatives for conventional energy generation apparatus. For mobile applications, the polymer electrolyte membrane fuel cell (PEM) is of particular interest. A proton-conductive polymer membrane is the main component in this type of fuel cell. Nafion®, which is a perfluorinated polymer with sulfonic acid side groups as produced by Dupont and similar products made by Asahi are still the market-dominating membrane materials for electrolyte membrane fuel cells.
Much research has been done with the aim to use other polymers as membrane materials in fuel cells. These polymers however are almost exclusively sulfonated materials whose proton conductivity is based on sulfonic acid groups.
In a relatively recent publication (M. Yamabe, K. Akijama, Y. Akatsuka, M. Kato, Novel phosphonated perfluorocarbon polymers, Eur. Polym. J. 36(2000) 1034–41), the synthesis of perfluorinated polymers with phosphonic acid side groups based on perfluorovinyloxy-substituted phosphonic acid as monomers is described which are then co-polymerized with tetrafluoroethylene and perfluoropropylvinyl ether. The use of such polymers in fuel cells is disclosed in U.S. Pat. No. 6,087,032.
Additional fuel cell membranes of polymers with phosphoric acid side groups are described for example in U.S. Pat. No. 5,422,411. These known polymers are made by radical homo- and co-polymerization of α, β, β-trifluorosstyrol-4-phosphoric acid esters and subsequent ester hydrolysis. The product obtained is a polystyrol with perfluorinated main chain, which functionalizes on the phenyl rings with phosphoric acid groups.
Additional polymers for use in fuel cells are described in U.S. Pat. No. 5,679,482. They are fluorine-free styrol copolymers with a mixed functionalization from sulfonic acid and phosphonic acid groups. The patent describes primarily SEBS-copolymers whose styrol units can be phosphonated by conversion with CICH2PO(OR)2. In the product, the phosphonic acid groups are not directly bound to the aromatic rings but by way of a methylene unit.
Also, U.S. Pat. No. 4,605,685 deals with the phosphonization of vinyl polymers which are used however for the development of cation exchanger membranes. The phosphonization of polybenzimidazole is described in U.S. Pat. No. 5,599,639; here lithium hydride is converted with BrCF2PO(OR)2. In the product obtained, the phosphonic acid groups are bound covalently to nitrogen atoms of the imidazole rings by way of CF2 units.
The first research for the use of phosphonated polymers in fuel cells was published by Cabasso (X. Xu, I. Cabasso, Preliminary study of phosphonate ion exchange membranes for PEM fuel cells, Polym. Mat. Sci. Eng. 68(1993) 120–121). For the manufacture of membranes, a phosphonated polyphenyl oxide and polymer blends prepared therefrom were used. Also in this polymer, whose synthesis is described in U.S. Pat. No. 4,073,754, the phosphonic acid groups are separated from the aromatic rings of the polymer main chain by the methylene units. These alkylene spacers reduce the chemical stability of the materials with respect to oxidation means and therefore detrimentally affect the long-term stability of the membranes during operation of a fuel cell.
An essential disadvantage of the initially mentioned perfluorinated materials such as Nafion® is the high price of these materials. With the use of fluorine-free aromatic polymers, it might be possible to lower the material costs without detrimentally affecting the chemical stability. The literature therefore includes numerous examinations directed at the sulfonization of polysulphones, polyether ketones, polyether ether ketones, polyamides and polyphosphazenes. However, all these materials have numerous disadvantages.
It is the object of the present invention to provide an arylpolymer with phosphonic acid side groups, which can be used for the manufacture of improved proton-conductive membranes. It is further an object of the invention to provide such a membrane.